Nitrogen Monitoring Techniques for Vegetable Crops - T Cooperative Extension Service Circular 579
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Nitrogen Monitoring Techniques
for Vegetable Crops
IC O
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Cooperative Extension Service • Circular 579
TE
College of Agriculture and Home Economics
Y
U
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IV T
E RSIPREFACE CONTENTS
Introduction .................................................................1
This circular is intended to serve as a practical guide
Background ..................................................................1
for growers who want to use a nitrate meter or nitrate
Equipment and supplies ...............................................2
test strips to improve their nitrogen fertilizer manage-
Procedure .....................................................................2
ment. Special emphasis is put on using these nitrogen-
Nitrate meter .......................................................2
monitoring techniques for chile pepper production.
Nitrate test strips .................................................2
The authors compiled the information during on-farm
Interpretation ...............................................................7
testing in southern New Mexico in 2000 and 2001.
Nitrate meter .......................................................7
Nitrate test strips .................................................7
Summary ......................................................................8
ACKNOWLEDGEMENTS References ...................................................................8
The authors gratefully acknowledge the contributions
of Raquel Chacon Miller, NMSU research aide, for her
work on procedures outlined in this circular. The authors
also acknowledge the generous cooperation of John Mor-
row, Dickie Ogaz and Marty Franzoy who allowed these
procedures to be tested on their farms.
Special thanks go to Denise McWilliams, Darrell Bak-
er and Margery Paroissien who reviewed the manuscript
and offered suggestions for improvement; and Cyndi
Gould who typed the final draft.Nitrogen Monitoring Techniques for Vegetable Crops
Robert F. Bevacqua and Tanya R. Cardenas1
INTRODUCTION gen was established in California for broccoli, celery,
sweet corn, lettuce, bell pepper, tomato and watermelon
Nitrogen (N) is the plant food most often applied to production. Currently, the nitrate meter is recommended
vegetable crops as a fertilizer. It is the nutrient respon- as a management tool for a similar range of vegetable
sible for green leafy growth. Insufficient nitrogen re- crops in Florida. The nitrate meter also is called an ion
sults in poor crop growth and low yields. In New meter, Cardy meter or sap tester.
Mexico, nitrogen fertilizer typically is dissolved in The procedures for monitoring nitrogen outlined in
water and applied during irrigation. this publication were adapted from methods devel-
Nitrogen also is the nutrient most likely to contami- oped in California and Florida. The procedures were
nate ground and surface waters. The nitrate form of tested in on-farm trials in southern New Mexico in
nitrogen (NO3-) is especially prone to causing contami- 2000 and 2001. The first year focused on developing
nation when excessive amounts are applied to crops. the steps for using the nitrate meter on chile pepper.
This molecule is weakly held by soil particles and, This was important because even though chile pepper
therefore, is highly mobile in the soil. This allows is the most economically important vegetable crop in
excess nitrate to move readily into groundwater or with New Mexico, it has never been the subject of a nitrogen
surface runoff. The result can be water pollution. monitoring investigation. In the second year, the study
One strategy for pursuing profitable yields while was expanded to include the nitrate meter and nitrate
avoiding water contamination is to only apply the test strips, also called “Quick Tests.” The goal was to
amounts of nitrogen that will be used by the growing compare these two field techniques for measuring
crop. This can be accomplished by using analytical nitrogen with the current practice of submitting samples
techniques that monitor nitrogen levels and, thus, to an analytical laboratory. Chile pepper and water-
avoid insufficient or excessive nitrogen fertilizer ap- melon were used as test crops. Watermelon was in-
plications. cluded because nitrogen-monitoring techniques for
This publication describes the procedures for using this crop, especially the nitrate meter, have been estab-
two techniques—a nitrate meter and nitrate test strips— lished in California and Florida. The results indicated
for monitoring nitrogen in chile pepper and watermelon that the nitrate meter readings were in closer agree-
crops. The meter measures nitrate-nitrogen (NO3-N) in ment with the laboratory analysis than those of the test
the plant’s petiole or leaf stem sap, and the test strip strips. The nitrate meter measurements were more
measures soil nitrate in the root zone. Both offer the strongly correlated with the laboratory analyses for
advantage of giving immediate results. Current practice watermelon than for chile pepper.
is to submit plant and soil samples to a laboratory for Using the nitrate meter on onion, the second most
analysis, which can take three to four weeks. important vegetable crop in New Mexico, is not recom-
mended at present because firm guidelines for sample
collection and interpretation are not available. Using
BACKGROUND nitrate test strips is recommended for monitoring soil
nitrogen in all vegetable crops, including onion.
The value of nitrate meters and nitrate test strips as The authors would like to share an important obser-
on-farm techniques for monitoring crop and soil nitro- vation made while developing the nitrate meter proce-
1
Extension vegetable specialist and research aide, Department of Extension Plant Sciences, New Mexico State University, Las Cruces, New Mexico.
1dure for chile pepper. Chile pepper is unlike most PROCEDURE
vegetable crops. There is very little sap in the petioles,
and what little is present is dark and dense. It is very Nitrate meter
difficult to obtain sufficient sap for testing. For this
reason, our recommendation for sample collection is Collect 30-40 leaves that form a representative
different from other vegetable crops. We recommend sample for the field in question. It is important that
that a sample include a large number of leaves, and the the petiole or stem be collected with the leaf (fig. 4).
sample material include both petioles and leaf blades. Select recently matured leaves from high on the plant
Thus, in collecting a representative sample of 30 to 40 that are free of disease and damage (fig. 5). Place the
leaves from a field for nitrate testing, it is important to leaves in a paper or plastic bag and transport them on
include petioles and leaves. Watermelon, on the other ice in a cooler where they are protected from the heat
hand, is typical of vegetable crops and has abundant sap (fig. 6). The selected leaves can be stored for up to
in the petiole. A sample is composed of a smaller eight hours in a refrigerator or on ice. Rinse the leaves
number of petioles, for example 10; leaf blades are with distilled water and blot dry with a paper towel.
excluded in the sampling. It is best not to take readings in areas of direct
sunlight or high temperatures (fig. 7). With a sharp
knife and on a cutting board, chop or dice the leaf
EQUIPMENT AND SUPPLIES blade and the petiole (fig. 8). Before using, calibrate
the meter with two standard solutions for nitrate-
Nitrate meter nitrogen, which are included in the nitrate kit. Load a
sample of petiole in a garlic press (fig. 9) and squeeze
Various supplies and devices are needed to operate three drops of sap onto the meter’s sensor (fig. 10).
the nitrate meter (figs. 1 and 2). Of these supplies and After the meter reading (ppm nitrate-nitrogen) has
devices, some can be purchased locally; others must be stabilized (30 to 45 seconds) record the value
ordered through a catalog. Supplies that can be pur- (fig. 11). After each use, rinse the sensor with dis-
chased locally include plastic sandwich bags, a sharp tilled water (fig. 12) and blot dry (fig. 13). Repeat the
knife and cutting board, distilled water, a garlic press, a procedure two or three times with the remaining
stopwatch or timer, paper towels, a pocket calculator sample if possible (fig. 14). Calculate the average of
and cooler. The Cardy nitrate meter kit by Horiba can be the readings for the samples (fig. 15).
ordered through a catalog from Spectrum Technologies
for approximately $360. Nitrate test strips
Spectrum Technologies With a soil probe, collect 10-12 soil cores (fig. 16)
23839 W. Andrew Rd. from the root zone to form a composite sample that
Plainfield, IL 60544 represents the area (fig. 17). Store the soil cores in a
Phone: (800) 248-8873 sealed plastic sandwich bag. When all soil cores have
Internet: www.specmeters.com been collected, mix the soil into a homogenous sample.
Fill a volumetrically marked centrifuge tube with
Nitrate test strip 30 mL of .01 M calcium chloride (fig. 18). Add the
soil sample to the tube until the solution level rises to
Various supplies also are needed to use the nitrate test 40 mL (fig. 19). Place a cap on the tube and shake the
strips (fig. 3). Most must be ordered through a catalog. sample vigorously until all soil clumps are thor-
Items available locally include plastic sandwich bags, oughly dispersed. Let the sample sit until the soil
stopwatch or timer and a pocket calculator. Catalog- particles settle and a clear zone of solution forms at
ordered supplies include a soil sampler, centrifuge tubes, the top of the tube (fig. 20). Sandy soil particles may
a tube rack, easy squeeze bottles, calcium chloride separate in a matter of minutes. However, clay soils
.01 M solution, and nitrate (NO3) test strips. EM Quant may take hours to settle. Dip a nitrate test strip into
nitrate (NO3) test strips can be ordered through a catalog the clear zone. Shake off the excess solution and wait
from Ben Meadows Company for approximately $40. 60 seconds for color to develop. Compare the color
with the color chart provided on the strip container
Ben Meadows Company (fig. 21).
P.O. Box 5277
Janesville, WI 53547-5277
Phone: (800) 628-2068
Internet: www.benmeadows.com
2Figure 1. Cardy nitrate meter. Figure 4. Representative sample.
Figure 2. Supplies and equipment. Figure 5. Selection.
Figure 3. Nitrate test strips. Figure 6. Storage and transporting.
3Figure 7. Take reading out of direct sunlight. Figure 10. Squeeze sap onto the sensor.
Figure 8. Chop or dice. Figure 11. Record the value of the reading.
Figure 9. Load sample in a garlic press. Figure 12. Rinse the sensor.
4Figure 13. Blot dry. Figure 16. Collect soil cores with a soil probe.
Figure 14. Repeat procedure. Figure 17. Soil cores.
Figure 15. Calculate the average. Figure 18. Fill centrifuge tube with calcium chloride.
5Figure 21. Compare with the color chart.
Figure 19. Add soil sample to the tube.
Figure 20. Let a clear zone form.
6INTERPRETATION Table 1. Guidelines for interpreting nitrate meter
results: sufficiency levels for nitrate-nitrogen
Nitrate meter in chile pepper petiole/leaf sap.
Growth Stage Concentration (ppm)
Guidelines for interpreting readings from a nitrate Vegetative growth 900-1,400
meter are generally expressed as nitrate-nitrogen First open flowers 800-1,200
values. For this reason, it is important that the meter Early fruiting 500-800
be calibrated and set to display readings in nitrate-
nitrogen.
The readings are interpreted by using three classifi-
cations: deficient, sufficient and excess. The classifica- Table 2. Guidelines for interpreting nitrate meter
tion determines whether or not the grower should apply results: sufficiency levels for nitrate-nitrogen
nitrogen fertilizer. “Deficient” indicates the crop may in watermelon petiole sap.
respond to immediate fertilization, so an application is Growth Stage Concentration (ppm)
recommended as soon as possible. “Sufficient” means Vines, 6 inches long 1,200-1,500
enough nitrogen is present to meet short-term crop Fruits, 2 inches long 1,000-1,200
needs; no fertilizer application is recommended at the Fruits, 50 percent mature 800-1,000
present time. “Excess” indicates fertilization should be At first harvest 600-800
postponed until retesting shows nitrate-nitrogen levels
have declined. Therefore, no fertilizer is recommended.
For example, sap was squeezed from a sample of
Table 3. Test strips yield a result expressed in
chile pepper petioles/leaves on June 14, during the
ppm nitrate. This is converted to ppm nitrate-
crop’s flowering stage. The meter showed a reading
nitrogen by dividing the result by a correction
of 700 ppm. This falls below the sufficient range of
factor based on soil texture and moisture.
800-1,200 ppm (table 1). Thus, 700 ppm is classified
Correction Factor
as deficient and an immediate application of nitrogen
Soil Texture Moist Soil Dry Soil
fertilizer is recommended.
Sand 2.3 2.6
Continuing with chile pepper example, specific
Loam 2.0 2.4
information is now presented on how monitoring tech-
Clay 1.7 2.2
niques typically are used to make decisions about
nitrogen fertilizer application. A common recommen-
dation for drip-irrigated chile peppers grown for a
green harvest in mid-August is to apply 200 lbs of Table 4. Guidelines for interpreting nitrate test strips
nitrogen per acre during the growing season. First, 40 results: sufficiency levels for nitrate-nitrogen
lbs of nitrogen per acre are applied to the seedbed in a in root zone soil for all vegetable crops.
preplant application. Second, beginning with the ap- Concentration (ppm) Interpretation
pearance of green flower buds in early June, 20 lbs of > 20 Excess
nitrogen per acre are injected into the drip system per 10 to 20 Sufficient
week for eight weeks. Thus, a total of 160 lbs of < 10 Deficient
nitrogen per acre is injected in eight weekly incre-
ments. These increments represent eight opportunities
for decision-making. When the nitrate meter readings, that are 2 inches long. The meter showed a reading of
for example, are interpreted as being sufficient or in 1,100 ppm. This falls into the sufficient range of
excess, a decision can be made not to apply the next 20 1,000 to 1,200 ppm (table 2). Enough nitrogen is
lbs of nitrogen. This reduces production costs, while present to meet short-term crop needs; an application
assuring that the crop’s nitrogen requirements are met. of fertilizer is not recommended.
Readings can be graphed throughout the growing
season to maintain nitrate-nitrogen levels in the suf- Nitrate test strips
ficient zone. For example, with chile pepper (fig. 22),
the sufficient concentration of sap nitrogen decreases Test strips yield a result expressed in ppm nitrate.
throughout the growing season. A sufficient concen- But guidelines for interpreting the results, such as for
tration for vegetative growth is 900 to 1,400 parts per the meter, are expressed as nitrate-nitrogen. This ini-
million (ppm), while it is 500 to 800 ppm for early tial result is converted to ppm nitrate-nitrogen by
fruiting (table 1). dividing the result by a correction factor based on soil
In a second example, watermelon petioles are texture and soil moisture (table 3.). For example, a
collected on July 10, when the vines have small fruit damp loam soil gives an initial test strip reading of 30
7Figure 22. Weekly nitrate meter readings (using the example of green chile pepper) can be graphed during the
growing season to maintain nitrate-nitrogen levels in the sufficient zone.
ppm nitrate. Using a correction factor of 2.0 (table 3) heat and light. Third, the interpretation guidelines
for a moist loam soil, the initial reading is converted to were developed for California and Florida not New
15 ppm nitrate-nitrogen. This result is interpreted by Mexico. Fourth, the chile pepper petioles are very
using guidelines (table 4) to assign the corrected read- dry so many leaves are required to make a sample
ing into one of three classifications: deficient, suffi- with enough sap for testing with a nitrate meter.
cient or excess. This classification is similar to that of
the meter results described above and determines
whether or not a grower should apply nitrogen fertil- REFERENCES
izer. Continuing with this example and using the guide-
lines (table 4), a test strip reading of 15 ppm nitrate- Cardenas, T. 2001. Nitrate testing in chile pepper,
nitrogen would be interpreted as sufficient. This means p. 22-23. In: R.F. Bevacqua and R. Phillips (eds.). Drip
there is adequate nitrogen in the root zone; no fertilizer irrigation for row crops. N.M. Coop. Ext. Serv.
application is recommended. Circ. 573.
Hartz, T.K, R.F. Smith, K.F. Schulback, and
SUMMARY M. LeStrange. 1994. On-farm nitrogen tests improve
fertilizer efficiency, protect groundwater. California
The nitrate meter and nitrate test strips enable grow- Agriculture 48(4):29-32, Univ. Cal., DANR.
ers to quickly measure nitrogen levels in the plant and in
the soil, respectively. The results are interpreted by Hartz, T. 2001a. Managing fertility in drip-irrigated chile
using guidelines to assign the readings into one of three production, p. 13-15. In: R.F. Bevacqua and
classifications: deficient, sufficient and excess. The R. Phillips (eds.). Drip irrigation of row crops. N.M.
classifications indicate whether or not nitrogen fertil- Coop. Ext. Serv. Circ. 573.
izer should be applied.
These nitrogen-monitoring techniques enable grow- Hartz, T. 2001b. Soil nitrate-nitrogen “Quick Test,”
ers to apply nitrogen fertilizer at the right time, helping p. 37-38. In: R.F. Bevacqua and R. Phillips (eds.). Drip
to ensure high yields without making unnecessary ap- irrigation of row crops. N.M. Coop. Ext. Serv. Circ. 573.
plications that could adversely affect the environment.
This also can save farmers money. Hochmuth, G.J. 1994. Plant petiole sap-testing for veg-
The principal advantage to these techniques is the etable crops. Univ. Fla. Hort. Sci. Dept. Circ. 1144.
ability to get immediate results. Other advantages are
that the equipment is portable and easily available Hochmuth, G.J., and D.N. Maynard. 1998. Vegetable
through catalogs. production guide for Florida. Univ. Fla. SP 170.
There are several disadvantages to using the meter
and test strips. First, the results, though close, are Maynard, D.N. 2001. Watermelons: characteristics, produc-
not as accurate as measurements from an analytical tion, and marketing. ASHS Hort. Crop Prod. Ser., Alexan-
laboratory. Second, the instruments are sensitive to dria, Va.
8To find more resources for your home, family, or business, visit the College of Agriculture and Home Economics on the
World Wide Web at www.cahe.nmsu.edu.
New Mexico State University is an equal opportunity/affirmative action employer and educator. NMSU and the U.S. Department
of Agriculture cooperating.
September 2002 Las Cruces, NM
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